1. Field of the Invention
This invention generally relates to methods and apparatus for subterranean fluid separation and removal. Certain embodiments relate to producing liquid phase hydrocarbon fluids and a negligible amount, if any, of water from a well even if a substantial amount of water is in the well or in the formation surrounding the well.
2. Description of the Related Art
As used herein, the term “hydrocarbon fluid” generally refers to a liquid phase hydrocarbon fluid such as crude oil. Many conventional techniques for hydrocarbon fluid production are known in the art. However, in certain circumstances, conventional recovery techniques are not economically viable since a relatively large fraction of the fluids recovered from the production zone include water or saline water. The water may be attributable to the reservoir such as with coning and channeling and/or to problems with a well such as casing leaks, cement channels, barrier breakdowns, or wells misdirected into high water zones. The produced water is usually separated from the recovered oil at the well head and must be treated prior to disposal or must be re-injected into disposal wells. In many cases, the cost of separating, treating, disposing, and/or re-injecting the produced water may increase the cost of producing oil such that oil production is no longer economically viable. For example, water re-injection disposal costs may be about $0.25 per barrel to about $0.50 per barrel. In addition, if the water must be trucked for disposal, the disposal costs can rise to about $1.50 per barrel.
In the case of oil wells, if a relatively large fraction of the fluids recovered from the production zone includes water or saline water, production of oil from such wells is often marginal. Therefore, such wells may be commonly referred to as “marginal wells.” If production from marginal wells is stopped, production from such wells is often not recommenced. Wells that are no longer in production may be commonly referred to as “inactive wells” or “abandoned wells.” Abandoned wells present several potential environmental problems because abandoned wells may pollute groundwater and may spill oil and salt water on the surface. Therefore, abandoned wells may also present a potential environmental hazard to humans, animals (i.e., commercial and non-commercial), and vegetation. In addition, abandoned wells may or may not be closed, or “plugged.” For example, although oil field companies are required to plug wells after a designated time period of no production is finished, depressed oil prices and the gradual decline of oil fields has forced some operators out of business before the wells are plugged. Abandoned wells that have not been plugged may pose an increased environmental hazard. Therefore, government funding is often used to cover the expense of plugging abandoned wells such that the risk associated with these wells may be reduced.
There are roughly 500,000 or more marginal or abandoned wells in the United States that are estimated to have the capacity to produce about 20% of the oil demand in the United States. Several methods are known in the art for extracting additional oil from previously developed oil fields that have reduced production and that may or may not have been abandoned. Such methods are commonly referred to as “stimulation treatments.” Examples of such methods include steam injection, hot oiling, and flushing the wellbore with certain chemicals. Such methods must be repeated periodically to maintain economically satisfactory production. As with other production methods, eventually, the stimulation treatments may cost more than the resulting produced oil thereby rendering the stimulation treatments ineffective or causing the wells to be abandoned.
However, stimulation treatments generally do not address the problems associated with water production from wells. Therefore, such treatments may not be effective for producing oil from a previously developed oil well or a formation having a relatively high water to oil ratio. However, many modem oil fields are being developed more efficiently than previously developed oil fields. For example, some efforts for efficiently developing new oil fields include attempts to reduce the production of water by selecting zones for well completion that do not have the potential of producing a large amount of water. However, such planning and design methodology cannot be applied to existing oil fields, marginal wells, or abandoned wells. Therefore, until oil can be produced from marginal or abandoned wells without producing a substantial amount of water as well, the production of oil from such wells is not economically feasible.
Other efforts have been made to develop technologies for keeping produced water from reaching the surface of the well. For example, polymer gels are used to block water from the wellbore or to improve the sweeping efficiency thereby reducing water production. Determining an appropriate polymer gel for such an application, however, may be very complicated. For example, identifying an appropriate polymer gel depends on correct identification of the water source, correct identification of the conditions in the well such as temperature, salinity, or fluid compatibility, and correct identification of sizing, placement, and application.
In another example, dual-completion water sinks designed to produce oil and water legs separately reduce differential pressure and coning in dual-action pumping systems. Since the dual-completion water sinks separate oil and water downhole and re-inject the water, such sinks reduce the costs of treating the water. There are, however, several disadvantages of dual-completion water sinks. For example, dual-completion water sinks produce oil containing some water and water containing some oil. Therefore, at least some water is produced to the surface and must be disposed. In addition, dual-completion water sinks may not be applicable for formations in which the oil and water legs are not in good pressure communication. Therefore, determining if the oil and water legs are in good pressure communication requires knowledge of the porosity and permeability relationships in the formation. In addition, designing a dual-completion water sink may be complicated because accurate sizing of the perforations and tubing is required such that the reduction of the water leg pressure does not exceed the reduction of the oil leg pressure. Furthermore, for a conventionally-completed marginal or abandoned well, the water sink must first drain any water saturation around the top completion. Such a draining process is a relatively slow process that requires considerable pressure drawdown.
Additional efforts have focused on other technologies for separating water and oil downhole. Downhole oil/water separation provides accelerated oil production in addition to reductions in operating expenses, water handling costs, and lifting costs. Two available technologies for downhole oil/water separation include gravity separation using rod pumps or enhanced gravity separation using hydrocyclones. Such technologies can be used in wells that have a relatively high water to oil ratio, relatively good mechanical integrity, sufficient remaining oil reserves, and a good injection zone (i.e., separation, reasonable pressures, and chemistry compatible with water). However, such technologies generally do not work with heavy oils (i.e., an American Petroleum Institute (“API”) gravity less than 10). In addition, the efficiency of the oil/water separation may vary depending on a number of factors such as mixture viscosity, temperature, differential density, inlet water concentration, sand concentration, and gas concentration.
Accordingly, it would be advantageous to develop an extraction method and apparatus in which oil is separated from water downhole in the vicinity of the production zone that does not produce a substantial amount of water from the well, that can produce oil from existing oil wells or new oil wells, that can produce oil from oil wells regardless of the conditions in the oil wells, and that can produce oil from oil wells regardless of the characteristics of the formation in which the wells are completed.